Finely divided refined kaolin is widely used as a pigment to provide a glossy, white, opaque surface finish on printing paper. In order to manufacture the refined kaolin pigments, several steps are required. These include particle size fractionation, purifying crude clay while the clay is in the form of a fluid deflocculated aqueous slurry, bleaching the clay while in a flocculated state, filtering the flocculated clay to remove liquid water and then dispersing the flocculated clay in water to form a high solids slurry that is marketed as such or is dried, usually in a spray dryer, to provide a dry refined pigment capable of being mixed with water to form a dispersed fluid suspension.
Conventionally, kaolin clay pigments are applied as aqueous "coating colors" which comprise a clay pigment, a dispersing agent for the clay, a suitable binder such as a polymer latex, starch, or mixtures thereof and other minor additives. The viscosity of the high solids suspension of the clay coating pigment must be sufficiently low to permit mixing and pumping. After the binder is added, the resulting coating color must also have suitable viscosity for handling and application to the paper sheet. In addition, the coating should impart good opacity, gloss, brightness and printability to the finished paper. The coatings are applied at high machine speeds and therefore require the use of high solids, typically 70%. A dispersing or deflocculating agent is present in the clay in order to improve the fluidity of the composition. Typical deflocculating agents include sodium condensed phosphate salts or sodium polyacrylates.
When clays are received by the paper manufacturer they are already highly processed. The crude clay has been fractionated, metallic debris has been removed, the brightness of the clay has been adjusted, and the clay has been washed in a drum filter to provide a washed filtercake. A clay which has been beneficiated to this extent is herein referred to as a "processed clay". The processed clay pigments are chemically modified to impart a surface charge to the particles which provides a repulsive force which keeps the particles in a dispersed state. The processed clay can be made anionic or cationic by the addition of a dispersing agent. Clays are usually anionically dispersed. Each particle of clay is chemically modified to carry a similar negative charge, which produces a repulsive force and keeps the particles in a dispersed state.
Conventionally, clay flocculation is induced by adding a flocculating agent which carries a charge opposite the surface charge of the pigment to the dispersion to cause the particles to attract and form agglomerates. Flocculation of the particles is induced by neutralizing or reducing the charge on the particles so they no longer repel one another but aggregate. This is known in the art as "charge reduction" flocculation.
Various methods of preparing a coating composition by adding a charge reduction agent to flocculate the clay are known in the art. U.S. Pat. Nos. 4,772,332 to Nemeh et al. and 4,859,246 to Sennett both teach a method for stabilizing a slurry of anionic clay pigments by adding a water-soluble cationic agent and at least one anionic sulfonate to flocculate the clay particles by neutralizing the clay particle's charge. The cationic agent is a polyelectrolyte flocculant and more specifically polyamides or quaternary ammonium polyelectrolytes.
U.S. Pat. No. 3,804,656 to Kaliski et al. teaches a method for stabilizing a clay suspension using charge alteration. A water soluble nonionic surface active agent, a cationic surface active agent, and a strong base are mixed to create a suspension. The suspension is added to a dispersion of particles to reduce the charge on the particles.
U.S. Pat. No. 4,569,768 to McKinley teaches a method for flocculating solids in a cationic clay by a charge reduction mechanism. The method comprises contacting the slurry with a water soluble polymer having an anionic character, and then with a water soluble polymer having a cationic character. This method clarifies an acidic slurry and is useful in mining operations.
The British Patent No. 1,497,280 to Portland Cement Manufacture's Limited teaches a clay treatment which is designed such that the clay dispersion can be introduced to the wet end of the paper machine whereupon, in the presence of the acid or hard water, the clay flocculates onto the fibers. This however, does not yield a colloidally stable dispersion in a flocculated clay. The central mechanism of flocculation is introducing the ionically dispersed clay to a high acidic environment which contains multivalent cations. The flocculation is caused by a combination of salt effects and charge reduction. This method could not be used in a coating. The presence of multivalent cations (hard water) would cause the entire coating to flocculate to a degree that it would not flow sufficiently to be pumped or applied to paper. In the Portland patent, the addition of the dispersion to the solution of cations forces flocculation. The anionic polymers are much higher in molecular weight than those used to produce a solid stable coating in the present invention. These polymers typically have molecular weights ranging from 4M to 5M and are used as retention aids. By using polymers with such high molecular weights, the coatings are too thick and do not flow.
D. I. Lee, "Coating Structure Modification and Coating Hold-Out Mechanisms", 1981 TAPPI Coating Conference Preprint, teaches a method for flocculating clay particles using different electrolyte combinations in a clay suspension to adjust the particle's charge. The electrolytes increase the ionic strength of the clay suspension and decrease the repulsion of particles. This change in the charge allows the attraction forces to dominate and creates particle stability.
European Patent 0,245,553 to Pratt et al. teaches a method for flocculating the particles in a coating slurry by adding a small amount of water soluble cationic polyelectrolyte flocculant to prepare a bulked clay pigment. The resulting flocculated product can be dispersed to form a high solids, clay-water slurry to be used as a coating composition or filler for paper.
The above-mentioned charge reduction and charge alteration methods for flocculating pigments have several drawbacks. Adding a charge reduction flocculating agent to a coating composition increases the composition's viscosity and limits the level of the flocculating polymer used. Traditional flocculation approaches used flocculants whose flocculating abilities were, in a sense, too intense causing undesirably high thickness, viscosity, and ionic properties for use as paper coatings. Also, they often do not have a high enough solids composition to be used as suitable paper coatings.
U.S. Pat. Nos. 4,943,342; 5,085,707; and 5,168,083 all assigned to Georgia Kaolin Company, Inc. teach "defined" clays. These references teach particle size adjustment by removing a portion of the fine fraction by physical or chemical means. These methods are used typically for coatings with opacifying pigments.
It is the applicant's discovery that by using a polymer which might be considered to be a poor flocculating agent and which does not reduce the charge on the particle, one can produce a colloidally stable suspension of flocculated pigment for use in paper. The clays can be used as fillers or in coatings. In accordance with the invention, fluid flow properties are improved, higher solids is achieved and there is less interaction with the other materials in the coating by the anionic or nonionic polymer.